1. Field of the Invention
The present invention relates to a gas pressure transducer employing a piezoelectric crystal or quartzcrystal resonator with its impedance variable dependent on a gas pressure applied thereto.
2. Description of the Relevant Art
It is known that there is a definite correlation between the pressure of a certain gas and the impedance of a piezoelectric crystal, i.e., a quartz-crystal resonator, placed in the gas while the piezoelectric crystal is in a state of series resonance. Once the relationship between the pressures of gases and the impedances of a piezoelectric crystal is determined, the pressure of a gas of a known composition can be derived from the impedance of the piezoelectric crystal. A gas pressure transducer based on the above principle has already been put to use.
To determine the impedance of a piezoelectric crystal, it is generally required to use an oscillator circuit which applies an AC signal to the piezoelectric crystal. Two types of oscillator circuits, i.e., a self-excited oscillator circuit and a separately excited oscillator circuit, are normally used.
The self-excited oscillator circuit is widely used in the field of watches, clocks, and the like for highly accurate frequency oscillation. However, this oscillator circuit design fails to quantitatively and highly accurately determine the impedance of a piezoelectric crystal in a state of series resonance dependent on the pressure of a gas, as in a gas pressure transducer. Therefore, any suitable gas pressure transducer with a self-excited oscillator circuit has not yet been put to practical use.
For the reason described above, conventional gas pressure transducers mainly utilize separately excited oscillator circuits. According to one simplest gas pressure transducer of such a configuration, an AC voltage having a series-resonant frequency that is generated by a frequency regulator is applied to a pressure sensor comprising a piezoelectric crystal, and a current which flows through the piezoelectric crystal and has a magnitude depending on a gas pressure applied thereto is processed by a current-to-voltage converter and a rectifier to display the detected pressure. However, the resonant frequency of the piezoelectric crystal varies as the applied gas pressure changes, and drifts even due to slight contamination to which the piezoelectric crystal is subjected. Furthermore, different piezoelectric crystals have different resonant frequencies. The gas pressure transducer of this design is not practical in that the frequency regulator should be operated at all times to seek the resonant frequency while the pressure is being measured.
Japanese Laid-Open Patent Publication No. 60 (1985)-201225 discloses a gas pressure transducer which employs a separately excited oscillator circuit to eliminate the aforesaid drawbacks. The disclosed gas pressure transducer has an ability to automatically lock onto or track the resonant frequency. The gas pressure transducer includes a PLL (phase-locked loop) circuit as follows: A frequency control circuit with its frequency controlled by an applied input voltage is connected to a piezoelectric crystal to apply an AC voltage thereto. The output signal from the frequency control circuit and an output signal produced by a current-to-voltage converter connected to the output terminal of the piezoelectric crystal are compared in phase, and an output voltage corresponding to any phase difference is applied through a low-pass filter to the frequency control circuit. The frequency control circuit therefore applies to the piezoelectric crystal an AC voltage having a frequency which is equal to the resonant frequency of the piezoelectric crystal. As a result, there is no need for the operator to seek the resonant frequency with a frequency regulator.
The above gas pressure transducer still has its own shortcomings resulting from the separately excited oscillator circuit, i.e., the PLL circuit. More specifically, since the piezoelectric crystal has an extremely high Q, it is necessary to search for the resonant frequency by frequency sweep when the power supply is turned on. The frequency control circuit requires special circuitry for such frequency sweep. The gas pressure transducer is not suitable for quick pressure measurement since it does not operate immediately, but with a certain time delay, after the power supply is turned on. A certain capture range is normally set in the frequency control circuit for seeking the resonant frequency. If the capture range were too wide, the circuit would be susceptible to extraneous noise. If the capture range were too narrow, the circuit would be unable to lock onto the resonant frequency in the event that the resonant frequency drifted. Another problem is that there is a delay in the response to a time-dependent change in the measured pressure because of the time constant of the low-pass filter in the PLL circuit. Reducing the time constant would also make the circuit susceptible to extraneous noise.